Surprisingly, there are quite a few prepaid data plans from major carriers in the US. For example, I'm using T-Mobile's 4G Monthly plan which provides me with 5GB of data for $30 a month, no contract.
You can actually get a nano sim from T-Mobile, all you need is the IMEI number from the sim in order to register for their 4G prepaid plan.
There are other carriers too, like boostmobile, cricket, virgin, etc.
Nano SIM is a new SIM size for iPhone 5, I doubt you can get it anywhere right now since iPhone 5 hasn't shipped yet. You probably were referring to Micro SIM which was introduced for iPhone 4.
You can actually get a nano sim from T-Mobile, all you need is the IMEI number from the sim in order to register for their 4G prepaid plan.
There are other carriers too, like boostmobile, cricket, virgin, etc.